Polyvinyl chloride (PVC) polymers are used in large volumes throughout the world to make various end use products. These polyvinyl chloride resins are normally produced and sold in the form of a free flowing powder made up of individual resin particles. The resin particles are then extruded or calendered into various shapes to form useful products such as pipe and house siding.
An important feature of the resin in extrusion operations is the shape of the resin particle. Assuming a constant particle size and porosity, the more spherical the shape of the resin particle the faster the extrusion rate. Also, a spherical shaped particle will give higher bulk density which results in savings in shipping costs. The higher the bulk density, the more pounds can be loaded into the same volume railcar. Current commercial resins vary in shape from a highly irregular shape resembling "popcorn" to irregular eliptical shapes resembling a distorted "egg".
Producers of polyvinyl chloride have long sought to produce a spherical shape particle. This desirable goal has been attempted in the past, with various degrees of success. One of the problems encountered was what is referred to in the art as excess "fines". In a suspension grade PVC, "fines" are defined as particles which are less than 52 micron in particle size. These fines tend to plug screens in drying equipment and they create dust because of their small size.
Another problem encountered in making spherical PVC particles is the lack of porosity of the resin. Porosity is needed to allow the resin to absorb plasticizers in end use applications.
The thickened aqueous suspension process used before this invention to make spherical PVC particles also results in increased polymer scale buildup on the internal surfaces of the polymerization vessel.
It is believed that the problems of excess "fines", increased polymer scale buildup, and low porosity are caused by the catalyst and secondary dispersants going to the water phase of the thickened reaction medium, rather than remaining in the vinyl monomer phase. In a charge procedure used heretofore, the water containing the thickening dispersant is charged to the reactor and then the vinyl monomer is added. The monomer being lighter will float on top of the water layer. The catalyst and secondary dispersants are then added to the reactor. Being heavier than the vinyl, these materials sink through the vinyl layer and go to the water layer.
U.S. Pat. Nos. 3,620,988; 3,706,722; 4,229,547; 4,360,651; 4,458,057; and 4,435,524 all relate to producing PVC resin having spherical shaped particles. Copending U.S. patent application Ser. No. 707,634 filed March 4, 1985, now U.S. Pat. No. 4,620,629, teaches a method to produce spherical PVC particles having a low glassy particle content.
It is a long sought after and desirable goal of the polyvinyl chloride industry to have a resin with spherical shaped particles and which also has low fines, good porosity and reduced polymer buildup on the internal surfaces of the polymerization reactor.